The present invention relates generally to optical, birefringent filters, and more particularly to a birefringent filter including a pair of birefringent crystals of differing lengths, between which is sandwiched a partial polarizer.
One type of prior art birefringent filter module includes similarly aligned, perfect, entrance and exit polarizers, between which is located a birefringent crystal having fast and slow orthogonal axes. The entrance and exit polarizers are rotatable together relative to the birefringent crystal to provide tuning. In response to an optical energy pulse impinging on the entrance polarizer, a pair of output pulses having spaced occurrence times are derived. The spacing between the output pulses is dependent upon the difference in the indices of refraction of the fast and slow axes of the crystal, the length of the crystal, and the speed of light in vacuum. A number of these modules have been cascade together, whereby successive modules include birefringent crystals having lengths related to each other in a geometric progression relative to the length of the module having the longest crystal. Such a birefringent crystal arrangement is frequently referred to as a Lyot filter.
A problem with the prior art Lyot filter is that the amplitude versus output frequency response of the filter includes many secondary maxima of significant amplitude. Thereby, if it is desired for the filter to handle a wide band pass, there is a tendency for significant energy outside of the desired band pass to be passed through the filter, to produce deleterious effects.